1. Field of the Invention
This invention relates to a process for manufacturing a piece that is made of composite material as well as a device for its implementation.
2. Description of the Related Art
Aeronautical construction tends to resort to composite materials for the production of certain parts of an aircraft, for example for producing panels that form the outside skin of the fuselage, so as to reduce the on-board weight.
The pieces that are made of composite material comprise a resin matrix that is reinforced by fibers. According to one widely used embodiment, the fibers come in the form of one or more fold(s) that may or may not be woven, pre-impregnated with thermosetting resin, and stacked on one another in a predetermined order and with predetermined fiber directions. As a variant, the fibers cannot be pre-impregnated. In this case, at least one resin film is used.
To ensure the consolidation of the piece that is made of composite material, it is necessary to increase the temperature to achieve the polymerization of the resin.
This invention relates more particularly to an operating mode that consists in using a compacting bladder and an autoclave for the polymerization. Such a manufacturing process that is applied to a part of an aircraft fuselage that is made of composite material is described in the document FR-2,894,869 or US-20090020645.
FIG. 1 shows a device of the prior art. The piece that is made of composite material that is connected to an airtight device 12 and that is covered by a compacting bladder 14 is shown at 10, whereby a joint 16 ensures the seal between the device 12 and the compacting bladder on the periphery of the piece 10. A draining fabric 18 is inserted between the piece 10 and the bladder 14. In addition, a non-perforated film 20, making it possible to let the gases pass through but retaining the resin during polymerization, is inserted between the piece 10 and the draining fabric 18. Means 22 are provided to put under vacuum the volume that is delimited by the device 12, the compacting bladder 14, and the joint 16, whereby said means 22, by way of example, comprise at least one channel 24 or at least one opening that is provided close to the peripheral joint 16.
This unit is then subjected to a polymerization cycle in an autoclave by applying external pressure on the order of 7 bar (8 bar of absolute pressure) and a relative vacuum on the order of −0.75 to −0.85 bar.
This thermal cycle essentially comprises a first phase 26 with a temperature on the order of 135 to 150° C. during which the resin is liquid, and then a second phase 28 with a temperature on the order of 180° C. during which the polymerization of the resin takes place.
As illustrated in FIG. 2, a relative vacuum on the order of −0.85 bar, always less than 0 and essentially constant, is generated at the piece 10.
The application of the vacuum in the volume that is delimited by the device, the bladder and the joint makes possible the degassing of the entire cycle, which is reflected by minimal air enclosed while the external pressure ensures the maximum compacting of the piece that is reflected by a better rate of fibers.
Hereinafter, porosity rate is defined as the ratio of the volume of the pore sizes to the volume of the piece×100.
The porosity rate is an essential criterion that makes it possible to characterize the quality of a monolithic piece that is made of composite material with a resin matrix.
In fact, despite the care taken, porosities form during polymerization, and the porosity rate is often close to the acceptable limit on the order of 3% in the affected zones, which affected zones are to show less than 1% of the surface area of the piece.
In the least unfavorable case, by means of calculations of structure and exemptions, the piece can be used.
When the calculations show that the piece cannot withstand the required stresses, said piece is discarded.
The fact that the porosity rate is generally close to the acceptable limit and that procedures for calculation and exemption are necessary in this case makes manufacturing times difficult to predict.
To reduce the risks of porosities at the surface of the piece in contact with the bladder, the document FR-2,921,295 proposes an improved drainage system that comprises a plate with openings that is inserted between the draining fabric 18 and the non-perforated film 20.